File handling using open(), read(), write() and close()
The previous examples of file handling deal with File Control Blocks (FCB). Under MSDOS v3.x (or greater) and UNIX systems, file handling is often done using handles, rather than file control blocks.
Writing programs using handles ensures portability of source code between different operating systems. Using handles allows the programmer to treat the file as a stream of characters.
#include <fcntl.h> int open( char *filename, int access, int permission );
The available access modes are
O_RDONLY O_WRONLY O_RDWR O_APPEND O_BINARY O_TEXT
The permissions are
S_IWRITE S_IREAD S_IWRITE | S_IREAD
The open() function returns an integer value, which is used to refer to the file. If un- successful, it returns -1, and sets the global variable errno to indicate the error type.
#include <fcntl.h> int read( int handle, void *buffer, int nbyte );
The read() function attempts to read nbytes from the file associated with handle, and places the characters read into buffer. If the file is opened using O_TEXT, it removes carriage returns and detects the end of the file.
The function returns the number of bytes read. On end-of-file, 0 is returned, on error it returns -1, setting errno to indicate the type of error that occurred.
#include <fcntl.h> int write( int handle, void *buffer, int nbyte );
The write() function attempts to write nbytes from buffer to the file associated with handle. On text files, it expands each LF to a CR/LF.
The function returns the number of bytes written to the file. A return value of -1 indicates an error, with errno set appropriately.
#include <fcntl.h> int close( int handle );
The close() function closes the file associated with handle. The function returns 0 if successful, -1 to indicate an error, with errno set appropriately.
ęCopyright B Brown. 1984-1999. All rights reserved.